More than a decade ago, a frenzied 5-day search for the Boston Marathon bombers left some lessons in its aftermath. One was that increasingly pervasive surveillance technology could help catch the culprits. Another was that amateur online sleuths on Reddit could not. But the intense search this week for a suspect in a Brown University shooting that killed two students and wounded nine other people turned the tables on those expectations. Only after a Reddit tipster cracked the case could police tap into mass surveillance of the suspect's car.